Cecilia,
Thank you for the screenshot and the emailed file! I understood the cause of the discrepancy you faced.
The DW report has multiple break-downs (Canal, Estragia, Publisher, etc.). I assume that you SUM Unique Visitors in the pivot table. The problem is that Unique Visitors have to be deduplicated which is impossible to do in Excel from such an export (imaging the same visitor came from Publisher X and Publisher Y. Then "1" will be shown next to each one, but in Total it was one UV, not two). This is why the Pivot shows inflated numbers of UV.
What you may want to do is to export the data exactly in the format shown in the Workspace freeform table. There should be Channel and Segment dimensions + Unique Visitors metric. In this case, the csv file should have exactly the same numbers as in the Workspace table.
Channel X - Segment A - 100 Unique Visitors
Channel Y - Segment A - 200 Unique Visitors
Channel X - Segment B - 150 Unique Visitors
etc.